1. Field of the Invention
The invention relates to the field of limiting the speed at which a computer may run and more particularly, the disabling of the operation of the computer when the input clock signal exceeds an upper limit.
2. Prior Art
In some computer systems, the need may arise to disable the computer when an input clock signal exceeds an upper limit. For instance, too fast a clock may cause problems in a system since some of the components in the system may not be able to operate at the higher frequency.
In the fabrication of microprocessors, it is common to find that some of the microprocessors are able to operate at higher frequencies than others even though all are fabricated with the same process. Process variations effecting, for example, the thickness of oxides can cause some wafers runs to yield parts that operate at much higher frequencies than in other wafers runs. Some companies test the upper frequency performance of newly fabricated microprocessors and sort them into "frequency" bins. Consequently, some microprocessors when sold, are graded (and labeled) for higher frequencies than others. The microprocessor may not function properly at frequencies above its labeled frequency, especially under stressful ambient conditions or marginal power supply potentials.
Companies have found that parts labeled with one frequency are sometimes relabeled after being sold by the manufacturer with higher frequencies, and then resold. These relabeled parts could end up in computer systems sold to operate at higher frequencies than the manufacturer intended. An innocent purchaser of such a computer could find that the computer operates well part of the time but fails at other times. The failures may be difficult to trace and if traced to the microprocessor, reflect badly on the manufacturer.
At least one manufacturer has incorporated a speed governor in its microprocessors which cause the microprocessors to fail if the clocking signals applied to them are higher than the frequency they were originally sold for. When this is done, the microprocessors can never be used in higher frequency application since it will never operate at the higher frequencies.
In co-pending application "Processor Clock Governor", Ser. No. 07/625,847, filed Dec. 11, 1990 and assigned to the Assignee of the present invention, a circuit is described for disabling a microprocessor when an improper frequency is used. A problem with this circuit is that the manner in which the "kill" signal is generated is itself subject to process variations and hence may not be settable with the precision desired.
As will be seen the present invention provides a circuit which is substantially independent of process variations and additionally provides other advantages.